This invention relates to a signal transmitter comprising a signal transmitting circuit for transmitting a signal propagating in the air (hereinafter referred to as "an air propagation signal", when applicable) and its power source which are mounted on a key which is to be inserted into the key cylinder. Moreover, this invention relates to a key cylinder device which is capable of detecting the presence or absence of the key.
Recently, some motor vehicles have been equipped with a so-called "wireless door locking device". The wireless door locking device is designed as follows: A signal transmitting circuit and its power source, namely, a battery are mounted on an ignition key, which can be inserted into the ignition key cylinder and the door key cylinder. The signal transmitting circuit is activated by the driver to transmit a radio wave signal. The radio wave signal thus transmitted is detected by received-signal discriminating means. When it is determined that the radio wave signal is the one predetermined for the door of the motor vehicle, an electric actuator is operated to automatically lock or unlock the door.
The wireless door locking device is convenient in practical use, because the door is locked or unlocked merely by operating an operating button to transmit the radio wave signal.
However, the wireless door locking device suffers from the following difficulty: When the power source, namely, the power of the battery is consumed up, it is no longer possible for the signal transmitting circuit to transmit the radio wave signal; that is, it is impossible to lock or unlock the door with the radio wave signal.
On the other hand, a key cylinder device, for instance, for the door of a motor vehicle which has means for electrically detecting the insertion of the key into the key cylinder or the removal of it therefrom; i.e., the presence or absence of the key, has not been put in practical use yet.
A key cylinder device of this type, being exposed outside the vehicle body, is liable to be adversely affected by external environmental conditions. Hence, it is rather difficult for the key cylinder device to include the aforementioned detecting means for electrically detecting the presence or absence of the key. It is not impossible for the key cylinder device to include the detecting means; however, the resultant key cylinder device including the detecting means is intricate in structure and accordingly bulky as well.